The present invention generally relates to gas cylinder springs and more particularly to a self-contained gas cylinder used to cushion the plenums of a press.
Gas cylinders of the type disclosed in this application typically employ a compressible gas such as nitrogen gas to resist the movement of a piston within a cylinder. These cylinders are generally of two basic types, threaded body and stand alone cylinders. Threaded body cylinders have a cylinder body that is open at one end with a piston rod extending out of the other end. The open end has external threads that permit the spring cylinder to be threaded into a manifold. The manifold generally has a number of threaded openings for receipt of a number of cylinder springs. A supply of nitrogen gas is supplied to the manifold and to the spring cylinders through their open ends.
Stand-alone cylinders are self-contained gas springs. They are charged with a quantity of compressible gas, such as nitrogen gas and then sealed. The stand alone springs are placed in various openings in the plenums of a press to cushion the presses movement.
Both types of gas springs work in the same manner. A piston reciprocates within a cylinder body against a compressible gas, such, for example, nitrogen gas. The cylinder is pre-charged with about 2,000 psi of nitrogen gas. When the piston is forced into the cylinder body, the nitrogen gas is compressed to a maximum pressure of about 3,000 to 5,000 PSI, depending upon the cylinder and piston size.
There are several problems in known gas cylinders. One problem is the springs create heat as they operate which increases wear of the various parts of the cylinder. As should be appreciated, excessive heat reduces the life of gas springs, particularly the life of the seals employed in the springs.
U.S. Pat. No. 4,74 1,518 discloses a stand alone cylinder that is believed to create large amounts of heat. The piston has an elongated bearing along a majority of the piston that engages the inner wall of the cylinder as the piston reciprocates. This bearing generates heat as the piston reciprocates, which causes the bearing to wear as well as other components and reduces the life of the cylinder.
Other problems result from the very tight tolerances required between the piston and the adjoining parts of the gas cylinder such as for example the bearings used to guide the piston. One reason for the need for tight tolerances is because of the type of seals employed and the need to prevent the seals from creeping or extruding between the piston rod and an adjoining part such as for example the guide bearing.
In the gas spring disclosed in U.S. Pat. No. 5,129,635, relatively little clearance is provided between each rod 16, 18 and its bearings 58, 84. With reference to column 3, lines 47 to 52 of the '635 patent, "[this] clearance is usually about 0.002 to 0.004 of an inch per side or a total clearance of 0.004 to 0.008 of an inch between the diameters of each rod and its associated bearings. To achieve these close tolerances, preferably the bearings are sized after being pressed into their housing." The need to size the bearings after being pressed into their housing is a difficult and expensive process. It also makes maintenance of the cylinder difficult and expensive as well. But it is necessary to have the tight tolerances because, without them, the seals can extrude, resulting in leaks around the seals and excessive wearing of the seals.